University of Groningen cis-Dihydroxylation and Epoxidation of Alkenes by Manganese Catalysts - Selectivity, Reactivity and Mechanism Boer, Johannes Wietse de IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2008 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Boer, J. W. D. (2008). cis-Dihydroxylation and Epoxidation of Alkenes by Manganese Catalysts - Selectivity, Reactivity and Mechanism. University of Groningen. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 27-09-2021 Chapter 1 Enantioselective epoxidation and cis-dihydroxylation catalysts The metal-catalysed enantioselective epoxidation and cis-dihydroxylation of alkenes is discussed. Special attention is given to typical reaction conditions employed, selectivities achieved, terminal oxidants used and the synthetic utility of these catalytic systems. Sections 1.1.2-1.1.4 of this chapter have been adapted from: J. Brinksma, J. W. de Boer, R. Hage, B. L. Feringa, Manganese-based Oxidation with Hydrogen Peroxide, in: Modern Oxidation Methods, J.-E. Bäckvall (ed.), Wiley-VCH, Weinheim, 2004, pp. 295-326. 13 Chapter 1 An overview of the more synthetically useful enantioselective catalytic systems for both the epoxidation and cis-dihydroxylation of alkenes reported to date, is provided in this chapter. Typical reaction conditions, enantioselectivities achieved, terminal oxidants used and their synthetic utility are discussed briefly for each catalytic system. This chapter is not intended to afford a detailed and comprehensive overview of all aspects of the oxidation catalysts discussed, for this the reader is referred to the various excellent reviews mentioned in the individual sections. For epoxidation, the focus is mainly on first row transition-metal catalysts and as a consequence catalysts based on Re1, Ru2 and W3,4 are not discussed. For cis-dihydroxylation both the Os- and Fe-based catalysts are discussed. The non-heme iron catalysts developed by Que et al.5 are discussed in more detail, due to their relevance to the Mn-tmtacn catalysts described in this thesis. 1.1 Epoxidation 1.1.1 Titanium The Ti-catalysed asymmetric epoxidation of allylic alcohols was first reported by Sharpless et al.6 in 1980 (for V-catalysed epoxidations, see ref. [7]). Treatment of an allylic alcohol t IV i with BuOOH in the presence of catalytic amounts of Ti (O Pr)4 and (S,S)- or (R,R)-dialkyltartrate affords the corresponding epoxides in high yield (50-90%) and with high ee (>90 %, Scheme 1.1).8,9 The most commonly used dialkyl tartrate ligands are diethyl- and diisopropyl tartrate, (S,S)- or (R,R)-DET and -DIPT, respectively. The catalyst 10 is sensitive to H2O and anhydrous reagents and conditions should be used. The presence 11 of molecular sieves generally enhances both the yield and ee of the reaction. i 5-10 mol% Ti(O Pr)4 1 2 1 2 R R 6-10 mol% (+)- or (-)-DET R R O OH 1.5 equiv. tBuOOH OH R3 R3 mol. sieves (4Å), CH2Cl2,-20°C IV i Scheme 1.1 Epoxidation of allylic alcohols mediated by Ti (O Pr)4 and (S,S)- or 8 (R,R)-diethyltartrate. IV The catalytically active species is believed to be a Ti 2 dimer containing two dialkyl tartrate ligands (Figure 1.1) and both the oxidant tBuOOH and the allylic alcohol coordinate to one of the TiIV centres.8 The coordination of the allylic alcohol positions the substrate in such a way that the oxygen is delivered to one enantiotopic face of the alkene and this is key to the high ee’s observed for this reaction. A large number of both allylic and homoallylic alcohols have been used as substrates, showing the versatility of this epoxidation catalyst for (natural product) synthesis.8,12 The necessity for the presence of this alcohol functionality, however, limits this catalyst to this class of substrates. 14 Enantioselective epoxidation and cis-dihydroxylation catalysts O E OR O R3 RO O E Ti Ti R1 O E O O R2 O EtO O tBu Figure 1.1 Active complex for Ti-tartrate catalysed epoxidation.8 Recently, Katsuki and coworkers reported a series of Ti-based catalysts which are not 13,14,15 sensitive to H2O and which can use H2O2 as oxidant. The ligands used were derived from the well-known salen ligands where either one or both of the imine functionalities are reduced (i.e. salalen or salan ligands, respectively, Figure 1.2). H H H NN NN TiIV TiIV O O O O O R R O 2 Ph Ph 2 R= IV IV 14,15 Figure 1.2 (Ti (µ-O)(salalen))2 and (Ti (µ-O)(salan))2 complexes. Typical reaction conditions employ 2-5 mol% of the Ti-dimer and a slight excess of H2O2 14,15 (30%) (1.01-1.5 equiv.) in CH2Cl2 at room temperature (Scheme 1.2). The yields are moderate to good (50-99%) and ee’s are typically between 75 and 99%. Substrates reported so far include aryl-substituted alkenes, such as styrene and 1,2-dihydronaphthalene, and aliphatic cis-alkenes.14,15 It is worth noting that several terminal alkenes could be converted to their corresponding epoxides with 70-85% ee. The active species is proposed to be a mononuclear TiIV-η2-OOH species based on CSI-MS (cold-spray ionisation mass-spectrometry) alone, where the TiIV centre acts as a Lewis acid to activate the 14 peroxide. O 2-5 mol% (Ti-salalen)2 or (Ti-salan)2 N H N O Ti O alkyl R 1.01-1.5 equiv. H2O2 (30%) alkyl R O CH2Cl2,r.t. O R=H,alkyl IV IV Scheme 1.2 Catalytic epoxidation by (Ti (µ-O)(salalen))2 and (Ti (µ-O)(salan))2 (left) and proposed catalytic active species (right).14,15 15 Chapter 1 1.1.2 Mn-porphyrins Both FeIII- and MnIII-porphyrins have been employed for the epoxidation of alkenes.16 These complexes are converted to their respective high-valent metal-oxo species by terminal oxidants such as iodosylbenzene and sodium hypochlorite (NaClO). Many chiral porphyrin derivatives have been prepared and moderate to good ee’s (up to 96%) have been obtained in several cases.17,18,19 The substrate scope tested is usually limited to styrene and a few derivatives thereof and best results are generally obtained with cis-alkenes such as cis-β-methylstyrene. However, while for high asymmetric induction during the interaction of the approaching alkene to the high-valent metal-oxo intermediate the chiral groups should be close to the metal-oxo moiety to afford a (rigid) chiral pocket, at the same time these groups should not be too close, since intramolecular oxidation (and finally inactivation) of the catalyst occurs.18 Catalyst stability is a serious issue with porphyrin-based oxidation catalysts, especially when the often tedious syntheses of the (chiral) porphyrin ligands are considered. N Cl N MnIII N N Figure 1.3 Example of chiral porphyrin epoxidation catalyst.20 Without additives, the use of H2O2 as oxidant leads to homolytic cleavage of the O-O bond of the intermediate hydroperoxo complex, resulting in the formation of hydroxyl radicals and as a consequence non-selective oxidation of the substrate occurs.16 However, when a nitrogen containing heteroarene such as imidazole coordinates axially to the MnIII-porphyrin, heterolytic cleavage of the O-O bond is promoted, yielding the V 21 catalytically active Mn =O intermediate. Although excess H2O2 (3-5 equiv.) was required, good yields (85-99%) have been obtained for the epoxidation of several alkenes using [MnIII(TDCPP)]Cl 1.1 (TDCPP: tetra-2,6-dichlorophenylporphyrin, Figure 1.4).22 The amount of imidazole used can be lowered to 1 equiv. with respect to (w.r.t.) the manganese porphyrin when also a small amount (1 equiv. w.r.t. catalyst) of carboxylic acid is used.23 When both the imidazole and carboxylic acid were attached to the manganese porphyrin (1.2) (Figure 1.4 and 1.5) up to 1000 t.o.n.’s have been obtained for several substrates, including cyclooctene and p-chlorostyrene, using 2 equiv. of H2O2 w.r.t. 24,25 substrate (Figure 1.5). A few examples of chiral porphyrins employing H2O2 are known, however, the ee’s obtained are low (ca. 30%).26,27 16 Enantioselective epoxidation and cis-dihydroxylation catalysts Cl Cl R1 Cl 1.1 :R1 =R2 =Cl:[MnIII(TDCPP)]Cl NNCl III 1 N Mn 1.2 :R =-O(CH2)5-N N N 2 2 Cl R R =-O(CH2)5CO2H Cl Cl Figure 1.4 Mn-porphyrins used for the catalytic epoxidation of alkenes employing 22,24 H2O2 as terminal oxidant. 0.1 mol% 1.2 O R1 R2 2equiv.H2O2 (30%) R1 R2 CH2Cl2 /H2O(pH5) 0°C R1 = alkyl, aryl 2 R = alkyl, H O O HO HO (CH ) (CH2)5 2 5 HO O O O H O III MnIII 2 2 Mn N O N O N (CH ) N (CH2)5 2 5 O O -H2O HO (CH2)5 O O MnV N O N (CH2)5 Figure 1.5 Epoxidation of alkenes catalysed by Mn-porphyrin 1.2 (see Figure 1.4).23,24,25 As structural and functional mimicks for porphyrin containing enzymes, such as cytochrome P450, both Fe- and Mn-containing metalloporphyrins are useful models to gain insight into the intriguing chemistry exhibited by these biologically relevant systems.